The present invention relates to imaging systems which enhance image quality by reducing noise which reduces contrast in images especially images obtained from turbid media, such as encountered in biological specimens, and especially dermatological tissue wherein keratin is present. Media, which are turbid, may be characterized by having a high rms refractive index variation and high scattering cross sections.
The invention is especially suitable for use in confocal microscopy and especially in laser scanning confocal microscopes, such for example as the Vivascope confocal scanning laser microscope which is sold by Lucid Technologies, Inc. of Henrietta, N.Y., U.S.A. and described in an article by M. Rajadhyaksha, et al. entitled xe2x80x9cIn Vivo Confocal Scanning Laser Microscopy of Human Skin, Melanin Provides Strong Contrastxe2x80x9d which appeared in the Journal of Investigative Dermatology, Volume 104, No. 6 pg. 1 (June 1995), and which is the subject matter of an article by M. Rajadhyaksha and James M. Zavislan which appeared in Laser Focus World, page 119, (Feb. 10, 1996), and in the hand held scanning laser microscope which is the subject matter of U.S. patent application Ser. No. 08/650,684 filed May 20, 1996 in the name of James M. Zavislan et al. The invention is also useful in optical coherence tomography or interference microscopy.
It has been discovered in accordance with the invention, that by illuminating a medium by beams having generally circular polarization in opposite senses (left and right handed circular polarization) images obtained from return light from an image plane or section within a specimen, by responding to circular dichroism and retardation, of the return light (circular dichroism and retardation is intended to include degrees of elliptical polarization), that image distortion, such a produced by scattering sites adjacent to the image plane or section, tends to be minimized or at least reduced to a constant value, while optical signals due to index variations and other optical activity within the image plane or section (region of interest) are actually detected. Thus correlated noise from scatterers, which produces optical distortion and especially speckle effects in the image, are reduced thereby enhancing the quality of the image. The focal region (image plane or section) may be at the surface of the specimen or embedded in the specimen. Noise due to scattering sites away from the focal region may occur, whether the region is at the surface or embedded in the specimen. The section being imaged, especially in imaging of biological tissue, is of the thickness of a cell, for example about five microns.
Regions adjacent to the section of interest may have an abundance of scatterers both behind and ahead of the section in the direction of propagation of the illuminating beam which is incident on the section. These potential scattering sources are illuminated by the same optical field that illuminate the region of interest. There is a finite probability that return light from these scatterers will pass through a confocal aperture and reach the detector as optical signals from which the image of the section of interest is constructed. The spurious return light may manifest itself as speckle in the image. The use of sheared circularly polarized beams, in accordance with the invention, has been found to reduce such distortion, and especially speckle distortion, thereby providing additional contrast and enhancing the image quality.
Laser scanning confocal microscopy provides image enhancement in that laser light beams are used, especially beams of a wavelength, such as in the infra-red range, which are maximally transmitted. The confocal aperture restricts the section which is imaged to the focal region. The probability, nevertheless exists that scattered light from regions away from the focal region will pass through the confocal aperture and produce noise, especially speckle, which distort and reduce contrast in the image which is detected. It has been proposed to use light restricted to one polarization state, but only for surface reflection reduction by eliminating the other polarization state. It has also been proposed to use sheared beams and differential interference contrast to enhance microscope images. Such beams have been obtained using Nomarski or Wollaston prisms and the technique of using such sheared beams has been referred to as Nomarski microscopy. See D. L. Lessor et al. xe2x80x9cQuantitative Surface Topography Determination by Nomarski Reflection Microscopyxe2x80x9d, Journal of the Optical Society of America, Volume 69 No. 2, pg. 357 (February 1979). Nomarski microscopy techniques have also been proposed for use in confocal microscopy. See C. J. Cogswell, xe2x80x9cConfocal Differential Interference Contrast (DIC) Microscopyxe2x80x9d, Journal of Microcopy, Vol. 165, Part I, pp. 81-101 (January 1992). Even with Nomarski techniques applied to confocal microscopy, noise distortion, which appears to emanate from scattering sites adjacent to the focal plane or image plane of interest, has not been minimized.
It is a feature of the present invention to further enhance image quality in imaging systems by utilizing circularly polarized beams focused on the image plane thereby obtaining noise reduction in the image, especially speckle noise which may be attributable to scatterers adjacent to the image plane.
The noise reduction system described herein has application to optical coherence imaging often referred to as optical coherence-domain reflectivity, optical coherence tomography or optical coherence microscopy. (See Schmitt et al, Optical characterization of dense tissues using low-coherence interferometry, SPIE, Vol. 1889, pps 197-211, July 1993.) In this imaging modality, a low-coherence source is used to illuminate a interferometer with a phase-modulated reference arm and a sample arm. In the sample arm a focussing objective directs light into a sample, often a turbid biological specimen. Only light which is scattered from a depth in the tissue that has equal optical path as the optical path of the reference arm constructively interferes at the detector to provide an electronic signal that represents the optical signal from the sample. This coherence requirement eliminates the need for a confocal pinhole to select the image plane inside the tissue. Optical coherence imaging however, suffers from the same deleterious effect of adjacent scatters as does confocal imaging. This effect is reduced, however, by the same polarization illumination and detection system previously described.
Accordingly, it is the principal object of the present invention to provide improved imaging systems, and especially imaging systems using confocal microscopy, and more especially laser scanning confocal microscopy.
It is a further object of the present invention to provide improved confocal microscopes and especially improved laser scanning confocal microscopes.
It is still further object of the invention to provide improved confocal laser scanning microscopes which provide images of biological tissue, and especially dermatological tissue.
It is a still further object of the inventor to provide improved instruments using optical coherence interferometry.
Briefly described, a system embodying the invention enables viewing of section of a medium. Light is received by and returned from the section and from sites adjacent to the section. The system utilizes a polarization separator, such as Nomarski or Wollaston prism, and a polarization retarder, such as a quarter wavelength plate, both operative on laser light which is incident on the medium and which are disposed successively in the direction of the incident light. The separator and retarder process the incident light into light which is polarized generally circularly and in opposite senses. This oppositely handed polarized light is incident on the medium in the section being imaged at spots which are spaced laterally nominally in the plane of the section of interest providing interference of light returned from the sites (scatterers) adjacent to the section being imaged. The image may be constructed in response to a polarization parameter, for example the degree of circular dichroism, of the return light.